1. Field of the Invention
This invention relates to zoom lenses suited to be used in video cameras or photographic cameras and, more particularly, a zoom lens whose zoom ratio is about 10 and whose F-number is about 1.8 with the bulk and size thereof being minimized, while still permitting maintenance of a good optical performance.
2. Description of Related Art
For the photographic camera or video camera, there have been many previous proposals for zoom lenses of a type in which a lens unit other than the front or first lens unit is made movable for focusing, or of the so-called "rear focus" type.
In general, as compared with the zoom lens whose first lens unit is moved to effect focusing, the zoom lens of the rear focus type has advantages that the effective diameter of the first lens unit becomes smaller so that it becomes easier to improve the compact form of the entire zoom lens, that close-up photography, particularly, supershort focusing, is carried out with ease, and further that, since the lens unit for focusing is smaller in size and lighter in weight, because a weaker driving torque suffices for moving it, rapid focus adjustment can be carried out.
Such a zoom lens of the rear focus type is disclosed in, for example, Japanese Laid-Open Patent Application Nos. Sho 62-24213 and Sho 62-247316, where the zoom lens comprises, in order from an object side, a positive first lens unit, a negative second lens unit, a positive third lens unit and a positive fourth lens unit, the second lens unit being moved axially to vary the focal length and the fourth lens unit being moved axially to compensate for the image shift with zooming. Focusing is performed by moving the fourth lens unit.
In Japanese Laid-Open Patent Application No. Hei 4-43311, a zoom lens comprises, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the third lens unit is composed of a positive lens and a negative lens in the form of the telephoto type, thus shortening the overall length of the third lens unit and subsequent lens units.
Also, in Japanese Laid-Open Patent Application No. Hei 5-60974, the third lens unit is composed of a positive lens and a negative meniscus lens concave toward the image side. The third lens unit is thus simplified in the construction.
In most cases, the use of the rear focus type in the zoom lens produces the characteristics described before. That is, the entirety of the lens system is minimized in bulk and size. Rapid focusing becomes possible. Further, close-up photography becomes easier to do.
In the case of the zoom lens which comprises, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, if attempts are made to shorten the overall physical length of the third lens unit and the subsequent lens unit by increasing the refractive power of the third lens unit, the zooming or focusing movement of the fourth lens unit is caused to increase unduly greatly. With zooming in the intermediate region of focal lengths, therefore, for a close object, problems arise in that the third and fourth lens units are caused to mechanically interfere with each other, or that, as the air separation between the third and fourth lens units has to be widened greatly, the total length is caused to become rather longer.
Also, in this instance, the variation of aberrations with zooming or focusing is increased. It is, therefore, very difficult to simultaneously fulfill the requirements of achieving improvements of the compact form and of maintaining a good stability of high optical performance throughout.